OFDMA-based resource allocation for wireless communication systems

Abstract

Multipath fading, shadowing, path-loss and time-variation are important phenomena in wireless communications. The technique of Orthogonal Frequency Division Multiplexing (OFDM) has been widely used to combat these detrimental effects in the past decades. Orthogonal Frequency Division Multiple Access (OFDMA) is a multiuser version of OFDM digital modulation, which is currently adopted in many international standards and is also a popular candidate for multiple access in future wireless systems. OFDMA is capable of allowing different subcarriers to be individually assigned to different users so as to enable simultaneous low data-rate transmissions and to achieve diverse Quality-of-Service (QoS) requirements. In addition, OFDMA can exploit both frequency domain and multiuser diversities to enhance the attainable system capacity. With dynamic resource allocation designed for OFDMA systems, the spectrum efficiency is expected to be further improved.

The main objective of this thesis is to devise efficient algorithms for OFDMA-based resource allocation in wireless communication systems, with joint consideration of system capacity, user fairness, low complexity and spectrum sharing, while trying to achieve controllable tradeoff among these concerns.

Chapter 1 gives a brief introduction to wireless communication systems and provides the fundamental principle in OFDMA-based Radio Resource Allocation (RRA). In Chapter 2, a typical downlink OFDMA system is presented first. Then, two sub-issues on partial feedback Channel State Information (CSI) and adjustable QoS are discussed via newly developed methods, which lead to significantly reduced CSI and satisfy diverse QoS requirements, respectively.

In Chapter 3, different utility-based resource allocation schemes are investigated for Multiple Input Multiple Output (MIMO) - OFDMA systems. The optimality of the system is reviewed, and two bargaining solutions are utilized to formulate efficient algorithms for flexibly controlling user fairness.

Chapter 4 jointly considers the direct and relaying paths in a relay-assisted OFDMA cellular system. In this system, a novel implementation adopting full-duplex relaying is proposed for relay-destination selection, subcarrier and power allocation. This implementation has significantly improved spectrum efficiency as compared to conventional half-duplex relaying mode. In addition, it enables effective controllability on the tradeoff between system capacity and user fairness.

In Chapter 5, we study two sub-issues for OFDMA-based Cognitive Radio (OCR) systems. Firstly, a novel spectrum sharing model is proposed for OCR. This model can dynamically allocate radio resources to secondary users with the cooperation of primary users so that the capacity of secondary network is maximized and the co-channel interference is minimized. The effect of Interference Temperature Limit (ITL) on the capacity of secondary network is also investigated, which shows that a properly selected ITL value can balance the performance between the primary and secondary networks. Secondly, with a fairness concern, Accessible Interference Temperature (AIT) is exploited to formulate an effective implementation for a simplified OCR model.

In the last Chapter, the contributions made in this thesis are summarized, and the possible extensions and future research are briefly outlined.